., C++ (gcc) - rextester

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//g++  5.4.0

#include <iostream>
#include <string>
#include <cstdio>

struct rangeiter
{
    int value;
    
    bool operator==(rangeiter const& other) const
    {
        return value == other.value;
    }
    bool operator!=(rangeiter const& other) const
    {
        return value != other.value;
    }
    int operator*() const
    {
        return value;
    }
    rangeiter& operator++()
    {
        value += 1;
        return *this;
    }
    rangeiter operator++(int)
    {
        value += 1;
        return rangeiter{value-1};
    }
};

std::string inttostr(int x)
{
    char buf[256];
    int len = snprintf(buf, sizeof(buf), "%i", x);
    return std::string(buf, buf + len);
}

template<typename I, typename T, typename FF, typename NF, typename LF>
auto collect(I begin, I end, T const& defvalue, FF first, NF next, LF last) -> decltype(first(*begin))
{
    if(begin == end)
    {
        return defvalue;
    }
    I it = begin;
    auto acc = first(*it);
    for(++it; it != end; ++it)
    {
        next(acc, *it);
    }
    last(acc);
    return acc;
}

template<typename I, typename T = typename std::remove_reference<decltype(*(I{}))>::type>
T sum(I begin, I end)
{
    return collect(
        begin, end,
        T{},
        [](T const& x){ return x; },
        [](T& acc, T const& x){ acc += x; },
        [](T& acc){});
}

template<typename I, typename T = typename std::remove_reference<decltype(*(I{}))>::type>
T sum_noref(I begin, I end)
{
    return collect(
        begin, end,
        T{},
        [](T x){ return x; },
        [](T& acc, T x){ acc += x; },
        [](T& acc){});
}

struct mystr: std::string
{
    template<typename... Ts>
    mystr(Ts&&... args)
        : std::string(std::forward<Ts>(args)...)
    {
        std::cout << "\tstring created\n";
    }
    
};

int main()
{
    enum
    {
        begin = 0,
        end = 5,
    };
    mystr names[end-begin];
    for(int i = begin; i < end; ++i)
    {
        names[i-begin] = " + " + inttostr(i);
    }
    std::cout
        << "names created\n";
    std::cout
        << "("
        << sum(names, names + (end - begin))
        << ") = "
        << sum(rangeiter{begin}, rangeiter{end})
        << std::endl;
    std::cout
        << "("
        << sum_noref(names, names + (end - begin))
        << ") = "
        << sum_noref(rangeiter{begin}, rangeiter{end})
        << std::endl;
}

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Compilation time: 0.43 sec, absolute running time: 0.12 sec, cpu time: 0.07 sec, memory peak: 3 Mb, absolute service time: 0,56 sec